Original Article
Fracture Resistance of Teeth Restored with Direct and Indirect Composite Restorations
Hassan Torabzadeh1, Amir Ghasemi2, Atoosa Dabestani3, Sara Razmavar4
1Associate Professor, Iranian Center for Endodontic Research, Research Institute of Dental Sciences, Shahid Beheshti University of Medical Sciences, Tehran, Iran
2Associate Professor, Dental Research Center, Research Institute of Dental Sciences of Shahid Beheshti University of Medical Sciences, Tehran, Iran
3Private Practice
4Reseach Fellow, Preventive Dentistry Research Center, Research Institute of Dental Sciences of, Shahid Beheshti University of Medical Sciences, Tehran, Iran
Corresponding author:
A. Ghasemi, Dental Research Center, Research Institute of Dental Sciences of Shahid Beheshti University of Medical Sciences, Tehran, Iran [email protected]
Received: 28 March 2013 Accepted: 23 June 2013
Abstract
Objective: Tooth fracture is a common dental problem. By extension of cavity di- mensions, the remaining tooth structure weakens and occlusal forces may cause tooth fracture. The aim of this study was to evaluate and compare the fracture resistance of teeth restored with direct and indirect composite restorations.
Materials and Methods: Sixty-five sound maxillary premolar teeth were chosen and randomly divided into five groups each comprising thirteen. Fifty-two teeth received mesio-occluso-distal (MOD) cavities with 4.5mm bucco-lingual width, 4mm pulpal depth and 3mm gingival depth and were divided into the following four groups. G-1:
restored with direct composite (Z-250, 3M/ESPE) with cusp coverage, G-2: restored with direct composite (Z-250) without cusp coverage, G-3: restored with direct com- posite (Gradia, GC-international) with cusp coverage, G-4: restored with indirect composite (Gradia, GC-International) with cusp coverage. Intact teeth were used in G- 5 as control. The teeth were subjected to a compressive axial loading using a 4 mm diameter rod in a universal testing machine with 1 mm/min speed. Data were analyzed using one-way ANOVA and Tukey tests.
Results: The mean fracture strength recorded was: G-1: 1148.46N±262, G-2:
791.54N±235, G-3: 880.00N±123, G-4: 800.00N±187, G-5: 1051.54N±345. ANOVA revealed significant differences between groups (p<0.05). Tukey test showed signifi- cant difference between group 1 and the other groups. There was no significant differ- ence among other groups.
Conclusion: Direct composite (Z-250) with cusp coverage is a desirable treatment for weakened teeth. Treatment with Z-250 without cusp coverage, direct and indirect Gradia with cusp coverage restored the strength of the teeth to the level of intact teeth.
Key Words: Tooth Fracture; Composite Resins; Bicuspid
Journal of Dentistry, Tehran University of Medical Sciences, Tehran, Iran (2013; Vol. 10, No.5)
INTRODUCTION
Tooth fracture is a common dental problem.
Many factors such as tooth anatomy contribute
to cusp fracture; however, cavity preparation procedures seem to be the major cause of most cuspal fractures [1-4].
Lopes et al. (1991) have shown that large intracoronal mesio-occluso-distal (MOD) preparation in premolar teeth reduces cusp stiffness to one third of the level of sound teeth [5]. Extending cavity dimensions, the remaining tooth structure weakens and occlus- al forces will cause more deformation in the cusps [6]. Studies that have used photoelastic analysis have demonstrated that occlusal cov- erage by onlays reduces the stress in the re- maining tooth structure [7].
On the other hand, studies have shown that the weakening effect of preparation can be alle- viated with the use of adhesive materials [8- 11]. It has been suggested that the adhesive nature of composite has the ability to bind the cusps and decrease flexion, which is the main cause of fractures in teeth restored with amal- gam [12,13].
Furthermore, composite has a lower elastic modulus than amalgam; therefore, more load is absorbed within the composite compared to amalgam. Composite, therefore, may transmit lesser load to the underlying tooth structure [14]. In an effort to overcome problems of primary polymerization shrinkage of direct restorations, various indirect techniques for producing composite restorations have been developed [15].
A new generation of composites is classified by Touati as second-generation laboratory composite, or ceramic optimized polymers (Ceromers) [16]. The manufacturers and re- search data claim that they provide enhanced flexural strength, increased elasticity and frac- ture resistance compared with that of direct composites [16-19].
Regrettably, specific information regarding the restoration priority of the large cavities with direct or indirect techniques is scarce; there- fore, to substantiate the present data and also to investigate the new aspects of the subject, this study was conducted to compare the com- pressive fracture resistance of large cavities restored with cusp coverage using direct and indirect resin composite.
MATHERIALS AND METHODS Selection and Preparation of Teeth
Sixty-five newly extracted (not more than two months) sound human maxillary premolars were used. Any calculus and soft tissue depo- sits were removed from the teeth using hand scaler (Gracey Curette, SG 17/18, Hu-Friedy, Chicago, USA). Each tooth was carefully ex- amined under light microscope (X10) for any existing enamel fissure or fracture. The teeth were stored in 0.5% chloramine solution for 48 hours, and then transferred to distilled wa- ter until preparation. The dimensions of the teeth were measured and taken into considera- tion for grouping. In order to minimize the in- fluence of size and shape variation of the teeth on the results, the height and width of each tooth was primarily measured bucco-lingually and mesio-distally with a digital caliper (Serial NO: 0026536, Mitotoya, Japan) with an accu- racy of 0.01 mm and then the teeth were clas- sified according to their size obtained from the following equations:
Tooth height = Buccal cusp edge to CEJ (Y1) + Palatal cusp edge to CEJ (Y2)/2
Tooth width = Mesiodistal width in height of contour area in palatal and buccal (X)
Tooth size = Tooth height/Tooth width
Subsequently, the teeth were distributed ran- domly into five groups of 13 each, and pre- pared as follows:
Group 1: Teeth restored directly with resin composite (Filtek Z-250, 3M/ESPE Co., St.
Paul, MN, USA) with cusp coverage.
Group 2: Teeth restored directly with resin composite (Filtek Z-250, 3M/ESPE Co., St.
Paul, MN, USA) without cusp coverage
Group 3: Teeth restored directly with resin composite (Gradia, GC-International) with cusp coverage
Group 4: Teeth restored indirectly with resin composite onlay (Gradia, GC-International) Group 5: Intact teeth (Control)
Templates were made for the teeth undergoing cavity preparation, using bleaching tray ma- terial to serve as guidance during reconstruc-
tion of the occlusal surface of the prepared teeth in groups 1 to 4. MOD cavities were prepared with round line angles and in order to have equal dimensions in all teeth (Fig 1), they were repeatedly checked with a digital caliper with an accuracy of 0.01mm (Serial No:
0020536, Mitutoyo, Japan). In groups 1, 2 and 3 that received direct filling, the walls of the cavity were prepared parallel, while in group 4; the walls were prepared with 15° diver- gence.
Preparations were carried out with diamond cylindrical bur (No 837L/012, Tizcavan, Iran) and each bur was replaced after 5 prepara- tions. All preparations were done under water cooling. Group 5 served as the unprepared control group. The composition of the mate- rials that were used in this study are presented in Table 1.
Direct Restoration Procedures
In groups 1 and 2, the teeth were restored with Filtek Z-250 composite (shade A3). First, the teeth were etched with phosphoric acid for 15 seconds, rinsed (30 seconds) and then dried (10 seconds). Two layers of adhesive (Single Bond, 3M/ESPE Co., St. Paul, MN, USA) were applied and gently air dried for 2 seconds, and then light-cured for 10 seconds.
The composite was placed using the incremen- tal technique and each layer was cured for 20 second saccording to the manufacturer’s in- struction. In group 3, G-bond (GC- International) was applied over the teeth, and then direct Gradia posterior composite (shade A3, Dentin) was placed. The adhesive was ap- plied with mini-sponge and dried with high pressure of air flow as instructed by the manu- facturer and cured for 10 seconds.
Table 1. Composition of the Material Used
Composition Batch
Number Company
Material
Filler: Zirconia, Silica (0.01-3.5
m), %60 Volume Matrix:BISGMA, UDMA, BISEMA 1370A3
3M Dental Products, st.
Paul, MN, USA Filtek
Z-250
Conditioner: Phosphoric acid %35. Silica thickner BISGMA:
Self priming adhesive, HEMA
A glycerolate dimethacrylate bisphenol dimethacrylate (HEMA-TMDI), Urethance
Copolymer alcheonic acid, Ethanol, Water 1105
3M Dental Products, st.
Paul, MN, USA Single
Bond
Filler: Silica, Pre-polymerized filler, Flouroaluminosilicate glass, %65 volume
Matrix: UDMA, Ethylen dymethacrylate 002005
GC Dental products corp.
2-285 Torllmatsu-Cho, Kasugai, Aichi, Japan Gradia,
Direct
Acetone, 4-Methacryloxyethyltrimellitic acid anhydride (4- META), Urethane Dimethacrylate (UDMA), Dimethacrylate component, Phosphoric acid ester, Water, Sillica Filler 002277
GC Corporation 76-1 Hasunuma-cho,itabashi-
KU, Tokyo, Japan G-Bond
Powder: Flouroalumino silicate
Liquid: Acrylic and maleic acid copolymer, HEMA, Water, Initiator
Treating agent: citric acid (%10), Ferric chloride (%2) and water
001409 GC Dental products corp.
2-285 torllmatsu-cho, lasugai, Aichi, Japan GC Fuji
Plus
Each layer of composite was light-cured for 20 seconds.
A matrix band was placed and then the restor- ative material was inserted in five layers as demonstrated in Fig. 2. Composites were po- lymerized with a halogen curing light (ARIALUX, Violet Spectrum Industries, Iran) with an intensity of 680-720 mW/cm2. Before restoring each cavity, the intensity of the appa- ratus was checked by the built-in radiometer.
Each layer was polymerized according to the manufacturer’s instruction, and after placing the last layer, the aforementioned templates were placed on the teeth to control the thick- ness of material covering the cusps. Then the restorations were finished and polished with diamond burs and discs (Soflex, 3M/ESPE Co., St. Paul, MN, USA), and then stored in water at room temperature for 24 hours.
Indirect Restoration Procedures
Impressions of the prepared teeth were taken with a condensational silicon rubber based material (Speedex, Asia Shimi Teb, Te- hran/Iran) and working dies were casted (Fuji Rock, GC Corp, Tokyo, Japan). Gradia sepa- rator (serial No: 0104231, Tokyo, Japan) was applied on dies and indirect composite (Gra- dia, GC Corp, Aichi, Japan) was placed using the layering technique. Each layer was poly- merized with pre-curing light (GC steplight SL-L, serial No: 03894, Tokyo, Japan) for 10 seconds; finally, they were placed in GC step- light SL-L (serial No: 03894, Tokyo, Japan) for 5 minutes in order to post-cure the compo- site. Each restoration was checked for margin- al fitness on both die and tooth. Restorations with an unacceptable fit, namely; those with a visible marginal opening and/or detectable with a probe (probe No.8, Dentsply LTD, UK) were excluded and new restorations were made. In the luting step, teeth were rinsed and dried, and then GC Fuji plus conditioner (GC Co., Tokyo, Japan) was applied on each tooth for 20 seconds. Teeth were rinsed completely and dried gently with air spray, but did not
dessicate. The cement (GC Fuji Plus) was mixed and placed on restorations as recom- mended by the manufacturer and subjected to a load of 2N that was inserted vertical to the occlusal surface of the tooth using a 2 kg weight for 5 minutes to standardize the cement thickness. The excess cement was removed in the gel phase. Samples were stored at room temperature for 24 hours. In each group, the samples were prepared in one day and were stored in distilled water at room temperature for 24 hours.
Testing
The teeth were mounted up to cervical, 1 mm below the cemento-enamel junction, in self- curing acrylic resin cylinders with 20mm height and 15mm diameter. The teeth were mounted in the universal testing machine (Model: 55144, Zwick/Roell, Germany) and were loaded to fail with a cross-head speed of 1mm/min using a 4mm diameter stainless steel rod that was placed in the midline of the tooth fissure. The results were analyzed using one- way analysis of variance (ANOVA) and Tu- key post-hoc test.
The mode of failure of the samples based on visual analysis was also studied using the fol- lowing criteria described by Burke et al. [20]:
Mode I- Minimal destruction of teeth;
Mode II- Fracture of one cusp, intact restora- tion;
Mode III- Fracture of at least one cusp, involv- ing up to one-half of the restoration;
Mode IV- Fracture of at least one cusp, in- volving more than one-half of the restoration;
Mode V- Severe fracture, involving tooth structure completely and/or longitudinal frac- ture.
RESULTS
The descriptive data are presented in Table 2.
One-way ANOVA revealed significant differ- ences between groups (P = 0.001); therefore, Tukey test was used to analyze the data (Table 3).
Table 2. Mean and Standard Deviation of Fracture Strength
*Mean ± SD (N) Number
Description Group
a1148.46 ± 262.67 13
Filtek Z-250 with cusp coverage 1
791.54 ± 235.86 13
Filtek Z-250 without cusp coverage 2
880.00 ± 123.90 13
Gradia, direct with cusp coverage 3
800.00 ± 187.44 13
Gradia, indirect onlay 4
a1051.54 ± 345.47 13
Control 5
Table 3. Tukey’s Post Hoc Test Results
Group I Group J P Value
1
2 0.004
3 0.049
4 0.005
5 0.846
2
3 0.884
4 1.00
5 0.061
3 4 0.917
5 0.382
4 5 0.075
Table 4. Mode of Failure in Test Groups
Group 1 Group 2 Group 3 Group4
Mode I 0 0 0 0
Mode II 0 0 0 0
Mode III 2 10 3 1
Mode IV 4 2 3 3
Mode V 7 1 7 9
Fracture resistance in the teeth restored with Filtek Z-250 with cusp coverage were higher than the other three groups (1148.46N±262.67), but similar to sound teeth (1051.54±345.47); however, sound teeth were similar to the four other groups (P= 0.001).
The specimen mode of failure is shown in Ta- ble 4.
A large number of the samples in groups 1, 3 and 4 showed modes IV and V (severe) frac- ture pattern. The fracture pattern of teeth in group 2 that were restored with direct restora- tion without cusp coverage showed less se- verity (mode III).
DISCUSSION
In the present study, the fracture resistance of human maxillary premolar teeth was eva- luated, as these teeth have shown a high inci- dence of fracture in clinic [21,22]. Vital teeth with conservative restorations are less suscept- ible to fracture than those with large restora- tions or with root canal treatment, regardless of the restorative material used [23]. Unlike some studies and for simulation of the clinical condition, large MOD cavities were prepared, so that the width of the isthmus was larger than one third of the intercuspal width [12,24].
According to some studies and based on the one-third rule, in the present study onlay de- sign was selected [25-28].
As an alternative to “external splinting”, a number of studies investigated the suitability of the adhesive technique in order to obtain an
“internal splinting” for strengthening of the weakened teeth. It was suggested that these adhesive restorations preserve tooth structure, and at the same time, provide esthetic and function [29].
Based on this idea, inlay design was selected for one group to compare with adhesive onlays to define how much resistance can be recon- structed in such cavity design; moreover, the effect of the direct and indirect method on fracture resistance was compared using Gradia direct and indirect.
The results revealed that fracture resistance of the teeth restored with Filtek Z-250 with cusp coverage (1148.46
262.67 N) was signifi- cantly higher than the teeth filled with Filtek Z-250 without cusp coverage (791.54
235.86 N). Along this finding, some studies reported that regardless of the type of material used, routine coverage of weak cusps can in- crease the fracture resistance of the restored tooth to an equal value of the non-restored tooth [30]. In addition, a study based on finite element analysis reported that large onlay res- torations were characterized by pure compres- sion stress at the transition line angle between the occlusal coverage and the vertical wall of the cavity that seems to be favorable. They declared that the compressive-type interfacial stress is able to prevent deboning [31]. This behavior contrasts with that of inlays that showed a majority of tensile interfacial stress challenging the adhesive bond. It is claimed that the amount of interfacial tensile stress was highly related to the elastic modulus of the material. It seems that Filtek Z-250 with a higher elastic modulus caused lower interfa- cial stress and consequently, it showed a high- er fracture resistance; however, there is a sig- nificant difference between the fracture resis- tance of the teeth restored with or without cusp coverage, emphasizing its importance [31]. The results showed that fracture resis- tance of teeth restored with Filtek Z-250 with cusp coverage was significantly higher than the teeth restored with direct Gradia with cusp coverage. According to the manufacturer’s data, the difference between these two groups may be the result of difference in the mechan- ical properties of the two materials that affect the fracture nature, such as fracture toughness, elastic modulus, flexural and tensile strength.
In addition, the aforementioned mechanical properties can also be influenced by the com- position of the material i.e. type of resin ma- trix, type and amount of filler. Although Gra- dia has a ceramic and pre-polymerized filler, a higher level and larger particles of filler, the
mechanical properties of this material is ad- versely affected by UDMA monomer. A high- er ratio of flexible urethane in addition to TEGDMA results in an extensive deformation before fracture. In UDMA, the long aliphatic segment in repeated units results in more flex- ibility and coiling of the chain. Despite the ability of “O-CO-NH” groups in UDMA in forming hydrogen bonds, easy breakage of these bonds with water may be an explanation for the low elastic modulus and the increased flexibility of chains as a result of a moderate change in temperature or stress [32]. On the other hand, Filtek Z-250 is formed from zirco- nia filler, BisGMA, BisEMA and flexible UDMA monomers. BisGMA has two aromatic rings that cause a lower cyclization (it has no effect on the improvement of mechanical properties) of pendant groups and a higher cross linking in polymer [33]. BisEMA mole- cule does not contain any hydroxyl groups, so unlike BisGMA, hydrogen bonds among the molecules of this monomer do not exist. This leads to an increased movement of the mole- cules resulting in a higher degree of conver- sion [34]. Therefore, it is predictable that Fil- tek Z-250 shows more suitable mechanical properties and higher fracture resistance com- pared to Gradia. Teeth restored with Filtek Z- 250 composite with cusp coverage have a sig- nificantly higher fracture resistance from those restored with Gradia composite onlays. The difference in fracture resistance between these two materials can be explained by that of Z- 250 and direct Gradia, due to a nearly equal composition of direct and indirect Gradia (manufacturer’s information). The only differ- ence between direct and indirect Gradia is the probable increase of DC after post curing in indirect Gradia composite, and even if this fact is approved, it is more likely that the increase in DC is not that significant to improve the fracture resistance.
Although in this study a significant difference was not observed between groups 3 and 4,
teeth restored with direct Gradia with cusp coverage, showed a higher fracture resistance compared to those restored with Gradia onlay.
This could be due to the nearly equal composi- tion of direct and indirect Gradia according to the manufacturer’s information. There are two points that should be considered; first, the post-curing effect of indirect Gradia on the probable increase of fracture resistance; and second, the bonding of direct and indirect Gradia composites to the tooth. Contradictory results concerning the effect of DC on me- chanical properties of material have been re- ported [35-38]. In this study, it is more likely that DC did not increase, or despite DC im- provement, mechanical properties and fracture resistance did not change.
Another fact that is considered to have influ- ence on the fracture resistance of onlay is the type of cement used. Resin-modified glass io- nomers have been suggested as a cementing agent for inlay/onlay restorations due to the ability to release fluoride, as well as the bond- ing ability to the tooth structure; hence, Fuji Plus (GC international, Tokyo/Japan) was used. In clinic, the final prognosis for the maintenance of a fractured tooth is closely re- lated to the position of the fracture line.
The location of the fracture depends on the ability of the tooth to distribute the energy im- parted by trauma evenly over the whole tooth body; therefore, minimizing the tension created. Whenever tooth strength weakens due to any pre-existing damage, the energy loaded by mastication forces or trauma can be easily transferred and localized in the root region. In this situation, if a fracture occurs due to load concentration in a specific part of the tooth, presumably, the line of fracture will be trans- ported to the root region.
Therefore, it seems that with a higher fracture resistance the mode of failure has changed from the favorable mode III (group 2) to the severe mode (V) (groups 1, 3, 4), which is un- repairable in clinic.
CONCLUSION
Reinforcement of the teeth might be achieved through the restorative procedure;
however, the quality of this reinforcement is related to the material and type of restoration (i.e. onlay and inlay).
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